Catalyst for producing methacrylic acid

ABSTRACT

A catalyst composition useful for the oxidation of unsaturated aldehydes, particularly the oxidation of methacrolein to produce methacrylic acid, comprises the combination of oxides of molybdenum, copper, phosphorus, antimony, calcium, tungsten and optionally arsenic in predetermined relative atomic ratios.

CROSS-REFERENCE TO OTHER APPLICATIONS

This is a continuation-in-part of U.S. Ser. No. 972,743 filed Dec. 26,1978.

PRIOR ART

This invention relates to a process and catalyst for the vapor-phaseoxidation with molecular oxygen of methacrolein to methacrylic acid.

It is well known that unsaturated acids, such as acrylic acid andmethacrylic acid, can be produced by the vapor-phase oxidation of thecorresponding unsaturated aldehydes by means of molecular oxygen in thepresence of a suitable oxidation catalyst. A variety of catalystcompositions have been proposed for this purpose. Many such compositionscomprise the oxides of molybdenum and phosphorus in association with theoxides of various other elements, both metallic and non-metallic.

For example, and with respect to the catalyst to be discussed hereafter,British Pat. No. 1,430,337 and U.S. Pat. No. 4,000,088 propose the useof a catalyst composition in which the oxides of molybdenum andphosphorus are combined with the oxides of antimony, and copper andoptionally with chromium. The catalyst does not contain calcium,tungsten, or arsenic.

In U.S. Pat. No. 4,045,478 the use of calcium in molybdenum-phosphoruscatalysts is taught. The catalyst lacks copper, tungsten and includesantimony and arsenic as optional ingredients.

Catalysts requiring the presence of arsenic include U.S. No. 4,051,179and 3,998,876. In '179 copper and vanadium are treated as alternatives,while an alkali metal must be included, but antimony and calcium areonly considered as optional ingredients. In '876 antimony and calciumare lacking, while copper, and tungsten are considered optional

Two additional patents in which arsenic is considered an optionalelement are U.S. Pat. No. 4,070,397 and 4,042,533. While tungsten isrequired in '397, calcium is lacking while copper and antimony areconsidered optional. However, in '533 copper and phosphorus areoptional, while antimony is lacking.

Despite the disclosures of the prior art, a catalyst of this type is notformulated merely by combining the many elements which have beendisclosed in the prior art. Instead, the catalyst performance must bedetermined experimentally at the expected operating conditions. Smallchanges in composition may be very important in achieving improvedcatalyst performance and particularly in optimizing the catalystcomposition to suit a specific reaction and set of operating conditions.The point is well illustrated by the improvements in catalystformulation to be described hereinafter.

It has been found that catalysts for oxidation of methacrolein tomethacrylic acid have the characteristic property of remaining stablefor a long period of time and then, without warning, of beginning arapid decline in activity. Consequently, an increase in the useful lifeof such catalysts has been sought, which has been achieved in thepresent invention by the addition of certain stabilizing ingredients.

SUMMARY OF THE INVENTION

It has been discovered that when using the catalysts to be described toproduce methacrylic acid by vapor phase oxidation of methacrolein, it ispossible to achieve both high activity and high selectivity for asignificantly improved useful life compared to catalysts which lack thestabilizing elements which have been added. The catalyst compositioncomprises oxides of molybdenum, copper, phosphorus, antimony, calcium,tungsten and optionally arsenic in predetermined relative atomic ratios.More specifically, the catalyst composition of the invention comprisesthe oxides of the above specified elements in the following atomicratios: Mo=12, Cu=0.05-3, P=0.1-5 Sb=0.01-0.6, Ca=0.1-6, W=0.01-3, andAs=0-3. The catalyst composition may be regarded either as a mixture ofoxides of the named elements or as oxygen-containing compounds of theelements or both.

The catalyst composition used in the process of the invention also maybe expressed by the following general formula:

    Mo.sub.a Cu.sub.b P.sub.c Sb.sub.d Ca.sub.e W.sub.f As.sub.g O.sub.h.

wherein a to h indicate the atomic ratio of each component and when a is12, b is 0.05-3, c is 0.1-5, d is 0.01-0.6, e is 0.1-6, f is 0.01-3, gis 0-3, and h has a value which is determined by the valence andproportions of the other elements in the catalyst.

When such a catalyst as has been described is in contact with avapor-phase mixture of methacrolein, molecular oxygen, steam, andnitrogen at temperatures in the range of 250°-400° C. and pressures inthe range of 0-5 atmospheres, excellent activity and selectivity to theproduction of methacrylic acid is obtained for a longer period of timethan for a catalyst lacking tungsten and optionally, arsenic.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 graphically displays the change of reaction temperature over aperiod of operation to compare the useful life of oxidation catalysts.

FIG. 2 shows the effect of varying the antimony content of catalystsaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Catalyst Composition and Preparation

The catalyst of the invention comprises oxides or oxygen-containingcompounds of molybdenum, copper, phosphorus, antimony, calcium, tungstenand arsenic in predetermined atomic ratios, as expressed in thefollowing general formula: Mo_(a) Cu_(b) P_(c) Sb_(d) Ca_(e) W_(f)As_(g) O_(h) wherein a to h indicate the atomic ratio of each componentand, when a is 12, b is 0.05-3, c is 0.01-5, d is 0.01-0.6, e is 0.1-6,f is 0.01-3, g is 0-3, and h is a value determined by the valence andproportions of the other elements in the catalyst. It has been foundthat the amount of antimony employed in the catalyst is particularlyimportant. As will be seen later, the addition of antimony in smallamounts provides significantly improved conversion of methacrolein andwith higher selectivity to methacrylic acid than when no antimony isused. However, further increase of antimony content causes a loss ofboth conversion and selectivity so that the amount of antimony should belimted to a value of about 0.6. The preferred value is about 0.3. Otherelements, which may be included in minor amounts in the catalystformulation in order to promote catalyst activity or selectivity andwithout losing the advantages to be shown for the principal formula, areconsidered to be within the scope of the invention. The catalystcomposition may be regarded either as a mixture of oxides of the namedelements or as oxygen-containing compounds of the elements or both. Asprepared and/or under reaction conditions, the catalyst may containeither or both forms and both are intended to be included within thephrase "mixtures of oxides."

The catalyst composition is preferably used in unsupported form, e.g. inthe form of pellets or other like compressed shapes of various sizes,although conventional supports could be employed instead. Thecomposition may be formed in conventional manner using techniques wellknown to persons skilled in the art. For example, compounds ofmolybdenum, copper, boron, phosphorus, antimony, calcium, tungsten andarsenic are dissolved in a small amount of water or other solvent, andthe solutions are then combined and evaporated to dryness, e.g. in arotary dryer. The several components can be introduced into solution inthe form of various salts or other compounds of convenient types and nospecific form for the catalyst precursors is necessary. The use ofammonium salts, halides e.g. chlorides, nitrates or acid forms of theelements, e.g. phosphoric acid, are, however, particularly suitable.Preferably, however, aqueous solutions are employed and water-solubleforms of the elements are used. In some cases the solutions may haveacids and/or bases added to them to facilitate dissolution of thecatalyst precursors. For example, acids such as hydrochloric or nitricacid, or bases such as ammonium hydroxide, can be used as desired. Theresulting powder from the evaporation is then thoroughly dried andpreferably screened to eliminate large particles which make it difficultto produce uniform compressed shapes, such as pellets. Typically, thepowder is passed through a 20-mesh screen. The powder is then mixed withan organic binder which can be of any conventional type, such aspolyvinyl alcohol, and the mixture is thoroughly dried and againscreened, typically to provide a 20-80 mesh size. The dried mixture isthen preferably combined with a lubricant, again of any conventionaltype, such as stearic acid or graphite, and compressed into the desiredshape, e.g. pelletized, the compressed shapes typically having heightsand diameters of 1/16 inch to 3/8 inch. Finally, the thus producedcatalyst composition is activated at high temperature for a prolongedperiod in accordance with conventional practice in this art. Forexample, the pellets are placed in an oven or kiln, or in a tube throughwhich air is passed, at an elevated temperature (e.g. 300°-500° C.,preferably 325°-450° C.) for at least ten hours. In a particularlypreferred activation step, the temperature is raised at the rate of 20°C. per hour to a maximum of 420° C., preferably 320°-400° C., and thistemperature is maintained for 16 hours.

It will be understood that the foregoing description regardingpreparation of the catalyst in a form suitable for use in a vapor-phaseoxidation reaction is merely illustrative of many possible preparativemethods, although it is a particularly suitable method and is preferred.

Methods of Operation

The catalysts described are generally useful for the production ofunsaturated acids by oxidation with molecular oxygen of unsaturatedaldehydes, although the reaction of methacrolein to form methyacrylicacid is of particular interest. Other possible starting materials arethe monoethylenically unsaturated aliphatic monoaldehydes of from 3 to 6carbon atoms, such as acrolein, crotonaldehyde, 2-methyl-2-butenal, andthe like, or mixtures thereof.

The reaction in which the catalyst compositions of this invention are ofparticular utility and in which they provide high conversions andselectivities involves contacting th catalyst with methacrolein andoxygen in the vapor phase, preferably also in the presence of steam anddiluents. When the catalyst of this invention is used in the vapor-phaseoxidation of methacrolein to form methacrylic acid, the oxidationconditions employed are those generally associated with this reaction,although the molar ratio of oxygen to methacrolein should be kept at ahigh value near the flammable range in order to obtain the longestuseful catalyst life. Once reaction is begun, it is self-sustainingbecause of its exothermic nature. A variety of reactor types may beemployed such as fluid or fixed bed types, but reactors having thecatalyst disposed inside a multiplicity of heat exchanger tubes areparticulary useful and convenient.

The gaseous feed to the reactor contains appropriate concentrations ofmethacrolein, oxygen and steam and usually an inert gas, such asnitrogen, is also present. The oxygen is usually added as such or asair, which may be enriched with oxygen. As mentioned, conventionaloxidation conditions can be employed but it is a feature of the catalystof this invention methacrolein can be present in concentrations of morethan 5 up to about 20 volume percent of the total feed with a preferredrange of more than 5 up to about 15 volume percent. In general at least6 volume percent of the aldehyde is used in the feed. The correspondingranges for oxygen are 3 to 15 volume percent, preferably 5 to 12 volumepercent and for steam up to 50 volume percent, preferably 5 to 35 volumepercent, the balance being the inert gas or gases.

The temperature of the reaction should, for best results, be within therange of from about 270° to 450° C., preferably 280°-400° C., and theoptimum temperature range is 290° to 325° C. Because the reaction isexothermic, means for conducting the heat away from the reactor arenormally employed to avoid a temperature increase which favors thedestruction of methacrolein by complete oxidation to carbon dioxide andwater. The reactor temperature may be controlled by conventional methodssuch as by surrounding the catalyst-containing tubes with a molten saltbath.

The reaction may be conducted at atmospheric, superatmospheric orsubatmospheric pressure. Preferably, however, pressures ranging fromatmospheric up to about 8 kg/cm² absolute, preferably up to about 6.3kg/cm² absolute, and most preferably up to about 4.5 kg/cm² absolute areemployed.

The unsaturated acid product may be recovered by a number of methodswell known to those skilled in the art. For example, the acid may becondensed, or scrubbed with water or other suitable solvents, followedby separation of the unsaturated acid product from the scrubbing liquid.The gases remaining after the acid-removal step are suitably recycled tothe reaction, if desired, preferably after removal of CO₂ byconventional means, e.g., absorption in aqueous carbonate solution.

The features of the invention will be more readily apparent from thefollowing specific examples of typical catalyst preparation and its usein the oxidation of methacrolein. It will be understood, however, thatthese examples are for the purpose of illustration only and are not tobe interpreted as limiting the invention.

COMPARATIVE EXAMPLE Example I

In 750 cc of water are dissolved 636 grams of (NH₄)₆ Mo₇ O₂₄.4H₂ O. Then21.7 grams of Cu(NO₃)₂.3H₂ O are dissolved in 100 cc of water, 79.2grams of Ca(C₂ H₃ O₂)₂ ×H₂ O are dissolved in 500 cc of water, 20.5grams of SbCl₃ are dissolved in a mixture of 30 cc of water, and 10 ccof concentrated HCl and 34.5 grams of H₃ PO₄ are dissolved in a mixtureof 100 cc of water. These solutions are fed to a rotary dryer of 4000 cccapacity and the mixture is evaporated to dryness at a temperature of140° C. The resulting powder is removed from the dryer and dried in anoven at 200° C. for 12 hours. The dried powder is screened through a20-mesh screen, a 4% aqueous solution of polyvinyl alcohol is added insufficient quantity to make a damp mixture and this mixture is dried at75°-80° C. until the moisture content falls to 2-4 wt. %. The driedmixture is then screened to 20-80 mesh size particles, and about 2-6% ofstearic acid powder is thoroughly mixed with it. The resulting mixtureis then pelletized to form pellets of 3/16 inch height and diameter inwhich the catalyst components molybdenum, copper, phosphorus, antimony,and calcium are present in the atomic ratios of 12, 0.3, 1, 0.3 and 1.5,respectively. The pellets are then activated in an oven by heating themgradually at a rate of 20° C. per hour to 380° C. and maintaining themat this temperature for 16 hours. The activated pellets have a densityof 0.97 gm/cc.

COMPARATIVE EXAMPLE Example 2

A 150 cc quantity of the catalyst composition is placed in a reactordefined by a 1/2"×90" stainless steel pipe, the reactor pipe beingfilled with 50 cc of inert filler (silicon carbide) below the catalystbed and 100 cc of the inert filled above the catalyst bed inconventional manner to insure uniform temperature contact with thecatalyst. Nitrogen-diluted mixtures containing methacrolein, oxygen andsteam are fed to the reactor at a pressure of 1.74 kg/cm² (absolute) andat a space velocity of about 1200 hr⁻¹. The term "space velocity" isused in its conventional sense to mean liters of gas (at standardtemperature and pressure) per liter of catalyst per hour. The feedcomposition is approximately, by volume, 6-7% methacrolein, 11-12%oxygen and 20% steam, the balance being nitrogen, determination beingmade on a wet basis. The reaction is run continuously and the exit gasis analyzed at intervals of several hours to give the overall effect ofa series of different runs. Analyses are carried out by means of gaschromatography and by infrared spectrography using conventionaltechniques.

For comparison of many catalysts, all of which are capable of providinga satisfactory yield of methacrylic acid, but which differ in theiruseful life, an accelerated aging test is carried out on the catalyst ofExample 1 and reported in the sole figure. The catalyst is tested undersevere conditions in order to obtain relatively quick determination ofthe catalyst performance by raising the operating temperature to thelevel needed to achieve at least about 80% conversion of methacrolein tomethacrylic acid. For commercial operation, the temperature mostsuitable for obtaining the best yield of methacrylic acid for a longperiod of useful catalyst life would be selected. As the catalystdeactivates, it is necessary to raise the operating temperature tomaintain a constant degree of methacrolein conversion. The upper limitof catalyst temperature is reached when the target of 80% conversion canno longer be obtained. This is generally found to be about 325°-330° C.At that point, the catalyst is no longer useful. While for commercialoperation a useful life of at least 2-3000 hours is desired, by means ofthe accelerated aging test, even durable catalysts can be fullydeactivated within about 100 hours, thus providing catalyst lifeinformation which might be obtained only after thousands of hours undermore conventional conditions.

Example 3

A catalyst corresponding to that of Example 1 is prepared by the sametechnique except that 38.1 grams of ammonium meta tungstate (NH₄)₆ H₂ W₂O₄₀.XH₂ O dissolved in 200 cc of water are included in the initialsolution to provide tungsten in a catalyst having the following nominalcomposition:

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Ca.sub.1.5 W.sub.0.5 O.sub.i

The catalyst also may contain boron at the ppm level since enoughammonium borate was included to provide 0.3 mols of boron. However,analysis of finished catalysts indicate that boron introduced into thesolutin in this manner is substantially lost, presumably by sublimationduring calcination, and if present, is below the level of detection bynormal analytical methods, typically gravimetric and atomic absorptionanalysis. The catalyst is tested under the conditions of Example 2 andthe results plotted in the FIG. 1, where it may be compared with theresults of Example 2.

While the use of calcium in itself provides a catalyst having improvedlife, as may be seen from another commonly assigned U.S. patentapplication, the addition of tungsten provides greatly increased usefullife. While the catalyst containing calcium has lost essentially all ofits activity in about 160 hours, and has an induction period duringwhich time no activity loss is experienced of about 90 hours, thecatalyst containing tungsten in addition to calcium has a useful life ofabout 260 hours an an induction period of about 165 hours.

Example 4

A catalyst prepared by the same technique previously used, but with theinclusion of arsenic in the catalyst, which has the following nominalcomposition:

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Ca.sub.1 W.sub.0.5 As.sub.0.5 O.sub.i.

This catalyst is tested with a feed gas containing 7 vol % methacrolein,12 vol % oxygen, 20 vol % steam, and the remainder nitrogen and at apressure of 1.74 kg/cm² (absolute). At 288° C. the catalyst converted75% of the methacrolein, with a selectivity of 76% to methacrylic acid.The catalyst of Example 3 tested under similar conditions gave 62%conversion of methacrolein with a selectivity of 70% to methacrylic acidat a temperature 285° C. Thus, arsenic is considered to be advantageous,since it improves the performance of catalysts containing tungsten.

Example 5

In order to demonstrate the importance of antimony in the catalyst ofthe invention, several catalysts were prepared according to Examples 3and 4, but the antimony content was varied. The catalysts were testedunder the conditions of Example 2 except that no attempt was made toreach 80% conversion of methacrolein, but instead a determination wasmade as to the lowest temperature at which a satisfactory conversion andselectivity could be obtained. The results are shown in the followingtable, in which the values obtained for catalysts of Example 4 (W plusAs) are given in parentheses adjacent those obtained for catalysts ofExample 3 (W alone).

    ______________________________________                                                Reactor    Methacrolein Selectivity % to                                      Temp. °C.                                                                         Conversion, %                                                                              Methacrylic Acid                              Sb Content                                                                            W (W + As) W (W + As)   W (W + As)                                    ______________________________________                                        0       298 (297)  30 (64)      35 (67)                                       0.3     285 (300)  60 (70)      70 (75)                                       0.5     295 (313)  48 (45)      59 (58)                                       ______________________________________                                    

The data from the table are plotted in FIG. 2 which makes clear thecritical importance of the antimony content in catalysts of thisformulation. For purposes of the figure, the conversion and selectivityfigures are multiplied to provide a percentage yield of methacrylic acid(MAA) from the methacrolein fed. The yield of methacrylic acid is ofparticular importance with regard to commercial use of such catalysts,since the by-products are of lesser value. The figure clearly indicatesthat addition of antimony raises the yield of methacrylic acidremarkably, but that the yield appears to be maximized between 0.1 and0.6 and that about 0.3 is a preferred value. However, it also appearsthat the presence of arsenic may strongly affect the catalystperformance when amounts of antimony less than about 0.3 are used. Forsuch catalysts the yield of methacrylic acid is less dramaticallyimproved by the addition of antimony, but the decline in yield is sharpafter the preferred value of 0.3 has been exceeded. The catalystcontaining only tungsten and no arsenic shows a more clearly definedpeak value of yield located again about a value of 0.3 antimony.

What is claimed is:
 1. A catalyst composition suitable for thevapor-phase oxidation of methacrolein to produce methacrylic acidconsisting essentially of the oxides of molybdenum, copper, phosphorus,antimony, calcium, tungsten, and optionally arsenic and having theformula Mo_(a) Cu_(b) P_(c) Sb_(d) Ca_(e) W_(f) As_(g) O_(h), where:a=12; b=0.05-3; c=0.1-5; d=0.01-0.6; e=0.1-6; f=0.01-3; g=0-3; andh=value determined by the valence and proportions of the other elementsof the formula.